Combination of optical elements

ABSTRACT

The present invention relates to a method of preparing a film that includes at least one layer of anisotropic polymer material. The method can include polymerizing a mixture of a filtered polymerizable mesogenic material.

This application is a continuation of divisional application Ser. No.10/367,722 filed Feb. 19, 2003, which is a divisional of U.S.application Ser. No. 09/230,335, now U.S. Pat. No. 6,554,605, which wasthe national stage of International Application No. PCT/EP97/03676,filed 11 Jul. 1997.

The invention relates to a combination of optical elements comprising atleast one optical retardation film and at least one broadband reflectivepolarizer, characterized in that the optical retardation film iscomprising at least one layer of an anisotropic polymer material havingan optical symmetry axis substantially parallel to the plane of thelayer, said optical retardation film being obtainable by polymerizationof a mixture of a polymerizable mesogenic material comprising

-   -   a) at least one reactive mesogen having at least one        polymerizable functional group,    -   b) an initiator,    -   c) optionally a non-mesogenic compound having two or more        polymerizable functional groups, and    -   d) optionally a stabilizer.

The invention further relates to a means to produce substantially linearpolarized light comprising a combination of optical elements asdescribed above. The invention also relates to an optical retardationfilm used in such a combination of optical elements and to a liquidcrystal display comprising such a combination of optical elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b show a display device according to preferredembodiments of the present invention.

FIG. 2 shows the retardation versus viewing angle of inventive opticalretardation films compared to a state of the art optical retardationfilm.

FIG. 3 shows the measurement setup according to example B of the presentinvention.

FIG. 4 shows the spectrum of a broad waveband reflective polarizer thatis used in combination together with an inventive optical retardationfilm in a special embodiment of the invention

FIG. 5 shows the relative luminance versus viewing angle for inventivecombinations of a broad waveband reflective polarizer and an inventiveoptical retardation film compared to a combination of a broad wavebandreflective polarizer with a state of the art optical retardation film.

FIG. 6 shows a method of rubbing a substrate for the use in the processof preparing an inventive optical retardation film.

The European Patent Application EP 0 606 940-A1 discloses a cholestericreflective polarizer that produces circular polarized light of a highluminance over a broad range of wavelengths. However, since for mostapplications, e.g. in liquid crystal displays, polarizers producinglinear polarized light are needed, EP 0 606 940 further describes thecombined use of the broadband cholesteric polarizer together with aquarter wave foil or plate (QWF), which transforms the circularpolarized light transmitted by the cholesteric polarizer into linearpolarized light.

The laid open WO 96/02016 discloses a linear polarizer consisting of acombination of the broadband cholesteric polarizer described above and aquarter wave plate comprising a stretched film of an isotropic polymerwith negative birefringence.

However, when a liquid crystal display comprising a cholestericpolarizer like those described in EP 0 606 940 and WO 96/02016 iswatched under an increasing viewing angle, its optical properties, likee.g. the luminance and the contrast ratio usually are deteriorating.

It has therefore been desired to have available an optical retardationfilm that, when used together with a broad waveband cholestericreflective polarizer, like e.g. those described in EP 0 606 940 and WO96/02016 as mentioned above, produces substantially linear polarizedlight and that improves the optical properties of the reflectivepolarizer over a wide range of viewing angles.

Optical retardation films have been described in prior art. Usuallyuniaxially stretched films of a prefabricated isotropic or LC polymerlike those described in the above mentioned WO 96/02016 are used forthis purpose.

Optical retardation films made of polymerized mixtures of reactivemesogens have also been mentioned. Research Disclosure, May 1992, p.411,No. 33799 describes an achromatic wave plate made of a stack of twolayers between glass substrates which under irradiation with light showsa net retardation of ¼ of the value of the wavelength of light incidenton the stack. Each of the layers is consisting of an anisotropic polymernetwork which is obtained by curing an oriented layer of a mesogenicdiacrylate.

However, polymerizable liquid crystalline compositions containing onlyone polymerizable compound as disclosed in the above mentioned ResearchDisclosure in general exhibit high or even very high melting points,which in turn requires high temperatures for alignment andpolymerization, which is a serious drawback when manufacturing suchlayers.

Furthermore, the process of manufacturing a quarter wave plate asdescribed in the above mentioned Research Disclosure is complicated, asit requires that the two layers are coated, aligned and cured in twoseparate steps. This is especially a disadvantage for mass production,since the first of the films is used as a substrate for producing thesecond film, which significantly increases the costs in case productionlosses occur when manufacturing the second layer, or requires moresophisticated production procedures and controls.

Furthermore, the above mentioned document only discloses the use ofglass substrates for the production of the quarter wave plate, but doesnot teach a method of producing of a quarter wave plate as a flexiblefilm with a large area, which is most desired for a large scaleproduction and for many applications.

Consequently there has been a considerable demand for an opticalretardation film that, when used together with a broad wavebandreflective polarizer, enhances the optical properties of the polarizerover a wide range of viewing angles, that is easy to fabricate in largescale as a flexible film with a large area and does not have thedisadvantages of the optical retardation films of prior art as discussedabove.

One of the aims of the present invention is to provide an opticalretardation film having these properties. Another aim of the inventionis to provide a combination of optical elements comprising such anoptical retardation film and a broadband reflective polarizer. Yetanother aim of the invention is a liquid crystal display devicecomprising a liquid crystal cell and such a combination of opticalelements. Other aims of the present invention are immediately evident tothe person skilled in the art from the following detailed description.

It has been found that these aims can be achieved by providing anoptical retardation film and a combination of optical elementscomprising such an optical retardation film and a broadband reflectivepolarizer according to the present invention.

The object of the invention is a combination of optical elementscomprising at least one optical retardation film and at least onebroadband reflective polarizer, characterized in that the opticalretardation film is comprising at least one layer of an anisotropicpolymer material having an optical symmetry axis substantially parallelto the plane of the layer, said optical retardation film beingobtainable by polymerization of a mixture of a polymerizable mesogenicmaterial comprising

-   -   a) at least one reactive mesogen having at least one        polymerizable functional group,    -   b) an initiator,    -   c) optionally a non-mesogenic compound having two or more        polymerizable functional groups, and    -   d) optionally a stabilizer.

In a preferred embodiment of the present invention, the bandwidth of thewavelength band reflected by the broadband reflective polarizer is atleast 100 nm.

In another preferred embodiment of the present invention the retardationof the optical retardation film is from 50 to 250 nm.

In another preferred embodiment of the present invention, thecombination of optical elements additionally comprises a compensationfilm comprising a layer of an anisotropic polymer material with ahomeotropic or tilted homeotropic orientation, the compensation filmbeing positioned adjacent to either side of the optical retardationfilm.

In another preferred embodiment of the present invention, thecombination of optical elements additionally comprises a linearpolarizer, arranged in such a manner that the optical retardation filmand, if present, the compensation film are positioned between thebroadband reflective polarizer and the linear polarizer.

Another object of the present invention is a means to producesubstantially linear polarized light comprising the following components

-   -   I) a combination of optical elements comprising at least one        optical retardation film and at least one broadband reflective        polarizer and optionally a linear polarizer and a compensation        film as described above,    -   II) a radiation source, and    -   III) optionally a diffuser adjacent to the radiation source,    -   wherein the components I to III are arranged in such a manner        that the broadband reflective polarizer of the combination of        optical elements I is facing the radiation source II or, if        present, the diffuser III.

Another object of the present invention is an optical retardation filmwhich is comprising at least one layer of an anisotropic polymer with anoptical symmetry axis substantially parallel to the plane of the layerand which can be used in the combination of optical elements asdescribed above and below, said optical retardation film beingobtainable by

-   -   A) coating a mixture of a polymerizable mesogenic material        comprising        -   a) at least one reactive mesogen having at least one            polymerizable functional group,        -   b) an initiator,        -   c) optionally a non-mesogenic compound having two or more            polymerizable functional groups, and        -   d) optionally a stabilizer        -   on a substrate or between two substrates in form of a layer,    -   B) aligning the polymerizable mesogenic material such that the        optical symmetry axis is substantially parallel to the plane of        the layer,    -   C) polymerizing said mixture by exposing it to heat or actinic        radiation,    -   D) optionally repeating the steps A), B) and C) at least one        more time, and    -   E) optionally removing the substrate or, if present, one or two        of the substrates from the polymerized material,

In a preferred embodiment of the present invention, the substrate ontowhich the polymerizable mesogenic material is coated in step B) is aplastic sheet or film.

In another preferred embodiment of the present invention, the alignmentof the polymerizable mesogenic material is achieved by directly rubbingat least one of the substrates onto which the polymerizable mesogenicmaterial is coated in step B).

In another preferred embodiment of the present invention, the mixture ofthe polymerizable mesogenic material comprises at least one reactivemesogen having one polymerizable functional group and at least onepolymerizable mesogen having two or more polymerizable functionalgroups.

In yet another preferred embodiment of the present invention, thereactive mesogens comprised in the inventive mixture of thepolymerizable mesogenic material as described above and below arecompounds of formula IP-(Sp-X)_(n)-MG-R   Iwherein

-   -   P is a polymerizable group,    -   Sp is a spacer group having 1 to 20 C atoms,    -   X is a group selected from —O—, —S—, —CO—, —COO—, —OCO—, —OCOO—        or a single bond,    -   n is 0 or 1,    -   MG is a mesogenic or mesogenity-supporting group, preferably        selected according to formula II        -(A¹-Z¹)_(m)-A²-Z²-A³   II        wherein    -   A¹, A² and A³ are independently from each other 1,4-phenylene in        which, in addition, one or more CH groups may be replaced by N,        1,4-cyclohexylene in which, in addition, one or two non-adjacent        CH₂ groups may be replaced by O and/or S, 1,4-cyclohexenylene or        naphthalene-2,6-diyl, it being possible for all these groups to        be unsubstituted, mono- or polysubstituted with halogen, cyano        or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to        7 C atoms wherein one or more H atoms may be substituted by F or        Cl,    -   Z¹ and Z² are each independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—,        —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single        bond and    -   m is 0, 1 or 2, and    -   R is an alkyl radical with up to 25 C atoms which may be        unsubstituted, mono- or polysubstituted by halogen or CN, it        being also possible for one or more non-adjacent CH₂ groups to        be replaced, in each case independently from one another, by        —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO— —OCO—, —OCO—O—, —S—CO—,        —CO—S— or —C—C— in such a manner that oxygen atoms are not        linked directly to one another, or alternatively R is halogen,        cyano or has independently one of the meanings given for        P-(Sp-X)_(n)—.

Another object of the present invention is a liquid crystal displaydevice comprising a liquid crystal cell and a means to producesubstantially linear polarized light comprising a combination of opticalelements, said combination comprising an optical retardation film asdescribed above and below.

The retardation of the inventive optical retardation film is preferablyranging from 50 to 250 nm, very preferably from 60 to 200 nm, mostpreferably from 70 to 170 nm.

The optical retardation film according to the present invention ispreferably used in a combination of optical elements together with abroadband reflective polarizer. When using this combination, light thatis substantially linearly polarized can be produced.

The bandwidth of the wavelength band reflected by the reflectivepolarizer according to the inventive combination of optical elements isat least 100 nm, preferably at least 150 nm, most preferably at least200 nm, ideally 250 nm or larger.

Preferably the bandwidth of the reflective polarizer is covering thespectrum of visible light.

In another preferred embodiment of the present invention, the opticalretardation film according to the present invention is used togetherwith a broadband reflective polarizer, wherein the retardation of theoptical retardation film is substantially 0.25 times a wavelengthreflected by the reflective polarizer, so that the optical retardationfilm serves as a quarter wave retardation film (QWF).

The term ‘a wavelength reflected by the reflective polarizer’ in thisconnection is indicating a wavelength of a value of the FWHM (full widthhalf maximum) of the wavelength band reflected by the reflectivepolarizer.

In a preferred embodiment of the present invention this wavelength isselected from the range of the centre wavelength ±95 %, in particular±70 %, most preferably ±50 % of the HWHM (half width half maximum, whichis 0.5 times the value of the FWHM) of the wavelength band reflected bythe reflective polarizer.

In another preferred embodiment of the present invention this wavelengthis ranging from the centre wavelength plus 50% of the HWHM to the centrewavelength plus 99% of the HWHM of the wavelength band reflected by thereflective polarizer.

In another preferred embodiment of the present invention this wavelengthis ranging from the centre wavelength minus 50% of the HWHM to thecentre wavelength minus 99% of the HWHM of the wavelength band reflectedby the reflective polarizer.

Another preferred embodiment of the present invention is characterizedin that the combination of optical elements additionally comprises acompensation film in order to compensate the viewing angle dependence ofthe phase retardation of light transmitted by the optical retardationfilm and/or the reflective polarizer. The compensation film can bepositioned adjacent to either side of the optical retardation film.

Preferably a compensation film is used of which the phase retardation isopposite in sign and substantially equal in magnitude to the phaseretardation of the reflective polarizer over a wide range of viewingangles.

Particularly preferably a compensation film is used that comprises alayer of an anisotropic polymer material with a homeotropic or tiltedhomeotropic orientation.

Light incident on the broadband reflective polarizer is transformed intocircularly polarized light. However, due to the angle dependence of thephase retardation of at least one of the optical elements of theinventive combination comprising the reflective polarizer, the opticalretardation film and optionally the compensation film a part of thelight passing through these optical elements will become ellipticallypolarized. This part of the light can lead to undesired reduction of thecontrast of the display.

Therefore in a preferred embodiment of the present invention a linearpolarizer is provided in the optical path of the display behind theoptical components of the combination mentioned above in order to blockthe part of light that is not ideally polarized.

As a linear polarizer a commercially available polarizer can be used. Ina preferred embodiment of the present invention the linear polarizer isa low contrast polarizer. In another preferred embodiment of the presentinvention the linear polarizer is a dichroic polarizer.

Preferably the linear polarizer is positioned such that the anglebetween the optical axis of the linear polarizer and the major opticalaxis of the inventive optical retardation film is in a range from 30 to60 degrees, especially preferably from 40 to 50 degrees.

The inventive optical retardation film comprises a layer of apolymerized mesogenic material and is characterized by a significantlyhigh birefringence. Furthermore, the optical properties of the opticalretardation film, like e.g. the birefringence, can be controlled byvariation of the type and ratio of the reactive mesogens in thepolymerizable material.

For a liquid crystal display comprising a broad band reflectivepolarizer and an optical retardation film of the state of the art, likee.g. a quarter wave film (QWF) made of stretched PVA, the luminance atnormal incidence (viewing angle 0°) and at low values of the viewingangle is increased compared to a liquid crystal display comprising thereflective polarizer alone without an optical retardation film.

However, as the display comprising the a broad band reflective polarizerand a QWF as mentioned above is viewed under an increasing angle, theincreasing phase retardation by the QWF itself causes a reduction to theluminance, coinciding with the value measured for the display comprisingthe reflective polarizer as a single component at a certain angle. Thisangle is referred to as the ‘cross-over angle’ α_(c).

When using an inventive optical retardation film instead of aconventional QWF in the liquid crystal display, the crossover angleα_(c) increases significantly. In other words, the brightnessenhancement, i.e. the increase of luminance at low viewing angles, thatwas achieved by using the reflective polarizer is now extended also tolarge viewing angles.

The cross over angle α_(c) of a display comprising a combination ofoptical elements comprising an optical retardation film and a broadbandreflective polarizer according to the present invention is preferably25° or larger, particularly preferably 30° or larger, very particularlypreferably 35° or larger in all directions of observation.

The optical retardation films according to the present inventioncomprise at least one layer of an anisotropic polymer having a symmetryaxis that is substantially parallel to the plane of the layer. The termsubstantially planar is indicating in the foregoing and the followingthat the optical symmetry axis of said layer is having a tilt anglerelative to the plane of the layer being in the range from 0 to 25degrees, preferably 0 to 15 degrees, in particular from 0 to 10 degrees.Especially preferred are tilt angles from 0 to 5 degrees, in particulartilt angles of approximately 0 degrees.

Another object of the present invention is a means to producesubstantially linear polarized light comprising a combination of opticalelements as described in the foregoing and the following andadditionally comprising a radiation source, which is positioned on theside of the reflective polarizer not facing the other optical elementsof the above mentioned combination.

As a radiation source preferably a standard backlight for liquid crystaldisplays, like e.g. a side-lit or a meander type backlight, can be used.These backlights typically comprise a lamp, a reflector, a light guideand optionally a diffuser.

The radiation source can also consist of a reflector that reflectsradiation generated outside the means to produce substantially linearpolarized light. The display device according to the present inventioncan then be used as a reflective display.

The function of the inventive combination of optical elements is furtherexplained by FIG. 1 a, which shows a display device according to apreferred embodiment of the present invention as an example that shouldnot limit the scope of the invention. The main direction of lightfollowing the optical path is from the left side to the right side. Thedisplay device 10 consists of a side-lit backlight unit 11 with a lamp12 a and a combined light guide and reflector 12 b, a diffuser 13 and apolarizer combination consisting of a reflective polarizer 14 comprisinga layer of a liquid crystalline material with a helically twistedmolecular orientation, the inventive optical retardation film 15,optionally a compensation film 16 and a linear polarizer 17. The figurefurther depicts a liquid crystal cell 18 and a second linear polarizer19 behind the display cell.

Light emitted from the backlight 11 is interacting with the molecularhelix structure of the reflective polarizer 14 with the result that 50%of the intensity of the light incident on the reflective polarizer istransmitted as circular polarized light that is either right-handed orleft-handed circular polarized depending on the twist sense of themolecular helix structure of the reflective polarizer, whereas the other50% of the incident light are reflected as circular polarized light ofthe opposite handedness. The reflected light is depolarized by thebacklight and redirected by the reflector 12 b onto the reflectivepolarizer 14. In this manner, theoretically 100% of the light of a broadrange of wavelengths emitted from the backlight 11 are converted intocircularly polarized light.

The main part of the transmitted component is converted by the inventiveoptical retardation film 15 into linear polarized light, which is thencompensated by the compensation film 16, if present, and beingtransmitted by the linear polarizer 17, whereas light which is notcompletely transferred into linear polarized light by the opticalretardation film 15, such as elliptically polarized light, is nottransmitted by the linear polarizer 17. The linear polarized light thenpasses through the display 18 and the second linear polarizer 19 toreach the viewer 20.

FIG. 1 b depicts a display device according to another preferredembodiment of the invention having essentially the same construction asthat shown in FIG. 1 a, with the modification being that the inventiveoptical retardation film 15 is placed behind the compensation film 16when looking from the direction of incident light.

As described above, the high efficiency of the reflective polarizer isachieved by making use of the light reflected by the reflectivepolarizer after it has been reversed, for example in the backlight unitof the display, and redirected back again to the polarizer.

One preferred embodiment of the present invention is characterized inthat the means to produce linear polarized light is comprising areflector in order to re-reflect circular polarized light reflected bythe reflective polarizer. This can be for example a metallic or anon-metallic reflector.

In case a metallic reflector is used the light coming from thereflective polarizer is re-reflected as circularly polarized light withopposite twist sense. This reflected circular polarized light is thencompatible with the molecular helix of the reflective polarizer and isfully transmitted by the reflective polarizer.

In case a non metallic reflector is used, the light coming from thereflective polarizer is depolarized and interacts again with thereflective polarizer as described above. Depolarization of the lightreflected by the reflective polarizer can also occur due to internalreflection and/or refraction in and/or between the optical components ofthe display.

In another preferred embodiment of the present invention the means toproduce substantially linear polarized light comprises at least onediffuser film or sheet situated between the backlight and the reflectivepolarizer in order to optimize the angular distribution of the lightincident on the reflective polarizer and/or to depolarize lightredirected onto the reflective polarizer by the reflector as describedabove.

The means to produce substantially linear polarized light according tothe present invention may also comprise one or more adhesive layersprovided to at least one of the components comprising the reflectivepolarizer, the optical retardation film, the compensation film, thelinear polarizer and the diffuser sheet(s).

The means to produce substantially linear polarized light according tothe present invention can further comprise one or more protective layersprovided to at least one of the components comprising the reflectivepolarizer, the optical retardation film, the compensation film, thelinear polarizer, the diffuser sheet(s) and the adhesive layer(s) inorder to protect these components against environmental influence.

The inventive optical retardation films are obtainable by coating themixture of a polymerizable mesogenic material on at least one substratein form of a layer, aligning the material and polymerizing the alignedmaterial. As a substrate for example a glass or quarz sheet as well as aplastic film or sheet can be used. It is also possible to put a secondsubstrate on top of the coated mixture prior to and/or during and/orafter polymerization. The substrates can be removed after polymerizationor not. When using two substrates in case of curing by actinicradiation, at least one substrate has to be transmissive for the actinicradiation used for the polymerization.

Isotropic or birefringent substrates can be used. In case the substrateis not removed from the polymerized film after polymerization,preferably isotropic substrates are used.

Preferably at least one substrate is a plastic substrate such as forexample a film of polyester such as polyethyleneterephthalate (PET), ofpolyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC),especially preferably a PET film or a TAC film. As a birefringentsubstrate for example an uniaxially stretched plastic film can be used.For example PET films are commercially available from ICI Corp. underthe trade name Melinex.

Planar alignment in the coated layer of the inventive mixture of thepolymerizable mesogenic material, i.e. an orientation wherein themesogenic material has a symmetry axis that has a low tilt anglerelative to the plane of the layer, can be achieved for example byshearing the material, e.g. by means of a doctor blade. It is alsopossible to apply an alignment layer, for example a layer of rubbedpolyimide or sputtered SiO_(x), on top of at least one of thesubstrates.

An especially preferred embodiment of the present invention ischaracterized in that planar alignment of the polymerizable mesogenicmaterial is achieved by directly rubbing the substrate, i.e. withoutapplying an additional alignment layer. This is a considerable advantageas it allows a significant reduction of the production costs of theoptical retardation film. In this way a low tilt angle can easily beachieved.

The term low tilt angle in connection with the aligned layer of themesogenic material before and/or after polymerization according to thepresent invention is indicating in the foregoing and the following thatthe mesogenic material has a symmetry axis with a tilt angle relative tothe plane of the layer that is preferably smaller than 10 degrees,especially preferably smaller than 5 degrees, in particular smaller than3 degrees and ideally substantially zero degrees.

Preferably a plastic film, in particular a polyester film, e.g. Melinex,or a TAC film are used as a substrate in this preferred embodiment.

It is also possible to use a polymer film as a substrate, which isannealed after rubbing near the glass transition temperature T_(g) ofthe polymer in order to reduce the tilt angle. For example, when using aMelinex film (T_(g) 140° C.) as a substrate, the substrate can be rubbedand subsequently annealed for 20 to 40 minutes at a temperature of about130 to 140° C.

When using an anisotropic substrate like e.g. a Melinex film, thealignment quality is depending on the rubbing angle, i.e. the anglebetween the major rubbing direction and the major optical symmetry axisof the anisotropic substrate. Preferably rubbing is carried outunidirectionally in a direction substantially parallel to the majorsymmetry axis of the substrate.

For example rubbing can be achieved by means of a rubbing cloth or witha flat bar coated with a rubbing cloth.

In another preferred embodiment of the present invention rubbing isachieved by means of a at least one rubbing roller, like e.g. a fastspinning roller that is brushing over the substrate, or by putting thesubstrate between at least two rollers, wherein in each case at leastone of the rollers is optionally covered with a rubbing cloth.

In another preferred embodiment of the present invention rubbing isachieved by wrapping the substrate at least partially at a defined anglearound a roller that is preferably coated with a rubbing cloth.

This method is exemplarily described by FIG. 6, that depicts a substrate1, like e.g. a plastic web, which is wrapped at an angle a around arotating roller 2, with the arrow indicating the moving direction of theweb 1. The roller 2 may also be covered by a rubbing cloth. An inventivepolymerizable mesogenic mixture being coated on the web 1 that wasrubbed by this method shows planar alignment of a high uniformity with avery low or even substantially no tilt.

As rubbing cloth all materials can be used that are known to the skilledin the art for this purpose. For example velvet of a commerciallyavailable standard type can be used as a rubbing cloth.

Preferably rubbing is carried out only in one direction.

The ability of the substrate to induce alignment in an inventivepolymerizable mesogenic composition coated on this substrate afterrubbing the substrate will depend on the process parameters of therubbing process, like the rubbing pressure and rubbing speed and, incase a rubbing roller is used, on the rotational velocity of the roller,the rubbing roller circumference and the tension on the substrate.

The rubbing length in the rubbing process according to the abovedescribed preferred embodiments is preferably from 0.2 to 5 meters, inparticular from 0.5 to 3 meters, most preferably from 1.0 to 2.5 meters.

Polymerization of the inventive polymerizable mesogenic mixture takesplace by exposing it to heat or to actinic radiation. Actinic radiationmeans irradiation with light, X-rays, gamma rays or irradiation withhigh-energy particles, such as ions or electrons. In particularpreferably UV light is used. The irradiation wavelength is preferablyfrom 250 nm to 420 nm, especially preferably from 320 nm to 390 nm.

As a source for actinic radiation for example a single UV lamp or a setof UV lamps can be used. When using a high lamp power the curing timecan be reduced. The irradiance produced by the lamp used in theinvention is preferably from 0.01 to 100 mW/cm², especially preferablyfrom 10 to 50 mW/cm².

The curing time is dependening, inter alia, on the reactivity of thepolymerizable mesogenic material, the thickness of the coated layer, thetype of polymerization initiator and the power of the UV lamp. For massproduction short curing times are preferred. The curing time accordingto the invention is preferably not longer than 30 minutes, especiallypreferably not longer than 15 minutes and very particularly preferablyshorter than 8 minutes.

The polymerization is carried out in the presence of an initiatorabsorbing the wavelength of the actinic radiation. For example, whenpolymerizing by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals that start thepolymerization reaction. As a photoinitiator for radicalicpolymerization a commercially available photoinitiator like e.g.Irgacure 651 (by Ciba Geigy AG, Basel, Switzerland) can be used.

It is also possible to use a cationic photoinitiator, when curingreactive mesogens with for example vinyl and epoxide reactive groups,that photocures with cations instead of free radicals. Thepolymerization may also be started by an initiator that initiates thepolymerization when heated above a certain temperature.

In addition to light- or temperature-sensitive initiators thepolymerizable mixture may also comprise one or more other suitablecomponents such as, for example, catalysts, stabilizers, co-reactingmonomers or surface-active compounds.

In a preferred embodiment of the invention, the polymerizable mixturecomprises a stabilizer that is used to prevent undesired spontaneouspolymerization for example during storage of the mixture. As stabilizersin principal all compounds can be used that are known to the skilled inthe art for this purpose. These compounds are commercially available ina broad variety. Typical examples for stabilizers are 4-ethoxyphenol orbutylated hydroxytoluene (BHT).

The polymerizable mixture according to this preferred embodimentpreferably comprises a stabilizer as described above at an amount of 1to 1000, especially preferably 10 to 500 ppm.

Other additives, like e.g. chain transfer agents, can also be added tothe polymerizable mixture in order to modify the physical properties ofthe inventive polymer film. For example when adding a chain transferagent to the polymerizable mixture, the length of the free polymerchains and/or the length of the polymer chains between two crosslinks inthe inventive polymer film can be controlled. When the amount of thechain transfer agent is increased, polymer films with decreasing polymerchain length are obtained.

In a preferred embodiment of the present invention the polymerizablemixture comprises 0.01 to 10%, in particular 0.1 to 5%, very preferably0.5 to 3% of a chain transfer agent. The polymer films according to thispreferred embodiment show especially good adhesion to a substrate, inparticular to a plastic film, like e.g. a TAC film.

As a chain transfer agent for example monofunctional thiol compoundslike e.g. dodecane thiol or multifunctional thiol compounds like e.g.trimethylpropane tri(3-mercaptopropionate) can be used.

In some cases it is of advantage to apply a second substrate to aidalignment and exclude oxygen that may inhibit the polymerization.Alternatively the curing can be carried out under an atmosphere of inertgas. However, curing in air is also possible using suitablephotoinitiators and high UV lamp power. When using a cationicphotoinitiator oxygen exclusion most often is not needed, but watershould be excluded. In a preferred embodiment of the invention thepolymerization of the polymerizable mesogenic material is carried outunder an atmosphere of inert gas, preferably under a nitrogenatmosphere.

To obtain polymer films with good alignment the polymerization has to becarried out in the liquid crystal phase of the polymerizable mesogenicmixture. Therefore preferably a polymerizable mesogenic mixture is usedthat has a low melting point, preferably a melting point of 100° C. orlower, in particular 60° C. or lower, so that curing can be carried outin the liquid crystalline phase of the mixture at low temperatures. Thisis simplifying the polymerization process as less heating of the mixtureis required and there is less strain of the mesogenic materials, thesubstrates and the production equipment during polymerization. This isof importance especially for mass production. Curing temperatures below100° C. are preferred. Especially preferred are curing temperaturesbelow 60° C.

The thickness of the inventive optical retardation film obtained by themethod as described above is preferably 0.2 to 10 μm, in particular 0.5to 5 μm, most preferably 1 to 3 μm.

In another preferred embodiment of the present invention the thicknessof the inventive optical retardation film is 8 to 30 μm, in particular10 to 20 μm.

In a particularly preferred embodiment of the invention the opticalretardation film is used together with a broadband reflective polarizerand optionally a compensation film. The optical retardation film may beconnected to the reflective polarizer and/ or the compensation film as aseparate optical element. Preferably, the reflective polarizer and/orthe compensation film and the optical retardation film are integrated sothat they form an individual optical element. This can be done forexample by laminating the optical retardation film and the reflectivepolarizer together and/or the compensation film after manufacturing theoptical retardation film.

The polymerizable mesogenic material can also be coated and cureddirectly onto a reflective polarizer which serves as a substrate, thussimplifying the production process.

Alternatively it is also possible that the polymerizable mesogenicmaterial is coated and cured onto a compensation film which serves as asubstrate.

In another preferred embodiment of the present invention, the broadbandreflective polarizer and/or the compensation film of the inventivecombination of optical elements are comprising a layer of an anisotropicpolymer material that is obtained by polymerizing an oriented layer ofreactive mesogens. Particularly preferably these reactive mesogens havea similar structure like the reactive mesogenic compounds of formula Ias described above and below.

Thus, when using an inventive optical retardation film together with abroadband reflective polarizer and/or a compensation film according tothis preferred embodiment, it is possible to adapt the opticalproperties of the optical retardation film to those of the reflectivepolarizer and/or the compensation film by using materials comprising inprincipal a similar type of compounds. In this way a combination of anoptical retardation film and a reflective polarizer and/or acompensation film with superior optical performance can be obtained.

In a preferred embodiment the polymerizable mixture comprises reactivemesogenic compounds having two or more polymerizable functional groups(multifunctional compounds). Upon polymerization of such a mixture athree-dimensional polymer network is formed. An optical retardation filmmade of such a network is self-supporting and shows a high mechanicaland thermal stability and a low temperature dependence of its physicaland optical properties.

Thus, for example inventive optical retardation films can be obtainedthat exhibit an excellent thermal stability of the optical retardation,which does not change significantly when heating the film up to 120° C.

In another preferred embodiment the polymerizable mixture comprises 0 to20% of a non-mesogenic compound with two or more polymerizablefunctional groups to increase crosslinking of the polymer. Typicalexamples for difunctional non-mesogenic monomers are alkyldiacrylates oralkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typicalexamples for non-mesogenic monomers with more than two polymerizablegroups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

By varying the concentration of the multifunctional mesogenic or nonmesogenic compounds the crosslink density of the polymer film andthereby its physical and chemical properties such as the glasstransition temperature, which is also important for the temperaturedependence of the optical properties of the optical retardation film,the thermal and mechanical stability or the solvent resistance can betuned easily.

The terms polymerizable or reactive mesogen, polymerizable or reactivemesogenic compound, polymerizable or reactive liquid crystal (compound)and polymerizable or reactive liquid crystalline compound as used in theforegoing and the following comprise compounds with a rodlike, boardlikeor disklike mesogenic group. These mesogenic compounds do notnecessarily have to exhibit mesophase behavior by themselves. It is alsopossible that they show mesophase behavior in mixtures with othercompounds or after polymerization of the pure mesogenic compounds or ofthe mixtures comprising the mesogenic compounds.

Preferably the reactive mesogenic compounds exhibit mesophase behavioron their own.

In a particularly preferred embodiment of the present invention, thereactive mesogens comprised by the mixture of the polymerizablemesogenic material are compounds of formula IP-(Sp-X)_(n)-MG-R   Iwherein

-   -   P is a polymerizable group,    -   Sp is a spacer group having 1 to 20 C atoms,    -   X is a group selected from —O—, —S—, —CO—, —COO—, —OCO—, —OCOO—        or a single bond,    -   n is 0 or 1,    -   MG is a mesogenic or mesogenity-supporting group, preferably        selected according to formula II        -(A¹-Z¹)_(m)-A²-Z²-A³-   II        wherein    -   A¹, A² and A³ are independently from each other 1,4-phenylene in        which, in addition, one or more CH groups may be replaced by N,        1,4-cyclohexylene in which, in addition, one or two non-adjacent        CH₂ groups may be replaced by O and/or S, 1,4-cyclohexenylene or        naphthalene-2,6-diyl, it being possible for all these groups to        be unsubstituted, mono- or polysubstituted with halogen, cyano        or nitro groups or alkyl, alkoxy or alkanoyl groups having 1 to        7 C atoms wherein one or more H atoms may be substituted by F or        Cl,    -   Z¹ and Z² are each independently —COO—, —OCO—, —CH₂CH₂—, —OCH₂—,        —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—, —OCO—CH═CH— or a single        bond and    -   m is 0, 1 or 2, and    -   R is an alkyl radical with up to 25 C atoms which may be        unsubstituted, mono- or polysubstituted by halogen or CN, it        being also possible for one or more non-adjacent CH₂ groups to        be replaced, in each case independently from one another, by        —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO— —OCO—, —OCO—O—, —S—CO—,        —CO—S— or —C≡C— in such a manner that oxygen atoms are not        linked directly to one another, or alternatively R is halogen,        cyano or has independently one of the meanings given for        P-(Sp-X)_(n)—.

Particularly preferred are polymerizable mixtures comprising at leasttwo reactive mesogenic compounds at least one of which is a compound offormula I.

In another preferred embodiment of the invention the reactive mesogeniccompounds are selected according to formula I, wherein R has one of themeanings of P-(Sp-X)_(n)— as given above.

Bicyclic and tricyclic mesogenic compounds are preferred.

Halogen is preferably F or Cl.

Of the compounds of formula I especially preferred are those in which Ris F, Cl, cyano, alkyl or alkoxy or has the meaning given forP-(Sp-X)_(n)—, and MG is of formula II wherein Z¹ and Z² are —COO—,—OCO—, —CH₂—CH₂—, —CH═CH—COO—, —OCO—CH═CH— or a single bond.

A smaller group of preferred mesogenic groups of formula 11 is listedbelow. For reasons of simplicity, Phe in these groups is 1,4-phenylene,Phe L is a 1,4-phenylene group which is substituted by at least onegroup L, with L being F, Cl, CN or an optionally fluorinated alkyl,alkoxy or alkanoyl group with 1 to 4 C atoms, and Cyc is1,4-cyclohexylene.-Phe-Z²-Phe-   II-1-Phe-Z²-Cyc-   II-2-PheL-Z²-Phe-   II-3-PheL-Z²-Cyc-   II-4-Phe-Z²-PheL-   II-5-Phe-Z¹-Phe-Phe-   II-6-Phe-Z¹-Phe-Cyc-   II-7-Phe-Z¹-Phe-Z²-Phe-   II-8-Phe-Z¹-Phe-Z²-Cyc-   II-9-Phe-Z¹-Cyc-Z²-Phe-   II-10-Phe-Z¹-Cyc-Z²-Cyc-   II-11-Phe-Z¹-PheL-Z²-Phe-   II-12-Phe-Z¹-Phe-Z²-PheL-   II-13-PheL-Z¹-Phe-Z²-PheL-   II-14-PheL-Z¹-PheL-Z²-Phe-   II-15-PheL-Z¹-PheL-Z²-PheL-   II-16

In these preferred groups Z¹ and z² have the meaning given in formula Idescribed above. Preferably Z¹ and Z² are —COO—, —OCO—, —CH₂CH₂— orCH═CH—COO—.

L is preferably F, Cl, CN, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃, COC₂H₅,CF₃, OCF₃, OCHF₂, OC₂F₅, in particular F, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃and OCF₃, most preferably F, CH₃, OCH₃ and COCH₃.

Particularly preferred are compounds wherein MG is selected from thefollowing formulae

wherein L has the meaning given above and r is 0, 1 or 2.

The group

in this preferred formulae is preferably denoting

furthermore

with L having each independently one of the meanings given above.

If R as given in formula I is an alkyl or alkoxy radical, i.e. where theterminal CH₂ group is replaced by —O—, this may be straight-chain orbranched. It is preferably straight chain, has 2, 3, 4, 5, 6, 7 or 8carbon atoms and accordingly is preferably ethyl, propyl, butyl, pentyl,hexyl, heptyl, octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy,or octoxy, furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, methoxy, nonoxy, decoxy, undecoxy, dodecoxy,tridecoxy or tetradecoxy, for example.

Oxaalkyl, i.e. where one CH₂ group is replaced by —O—, is preferablystraight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or3-oxabutyl (=2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or5-oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-,7-, 8- or 9-oxadecyl, for example.

In addition, mesogenic compounds of the formula I containing a branchedgroup R can be of importance as comonomers, for example, as they reducethe tendency towards crystallization. Branched groups of this typegenerally do not contain more than one chain branch. Preferred branchedgroups are isopropyl, isobutyl (=methylpropyl), isopentyl(=3-methylbutyl), isopropoxy, 2-methylpropoxy and 3-methylbutoxy.

P in formula I is preferably selected form CH₂═CW—COO—, WCH═CH—O—,

or CH₂═CH-Phenyl-(O)_(k)— with W being H, CH₃ or Cl and k being 0 or 1,

P is particularly preferably a vinyl group, an acrylate group, amethacrylate group, a propenyl ether group or an epoxy group, veryparticularly preferably an acrylate or methacrylate group.

As for the spacer group Sp in formula I, Ia and Ib all groups can beused that are known for this purpose to the skilled in the art. Thespacer group Sp is preferably linked to the polymerizable group P by anester or ether group or a single bond. The spacer group Sp is preferablya linear or branched alkylene group having 1 to 20 C atoms, inparticular 1 to 12 C atoms, in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by —O—, —S—, —NH—, —N(CH₃)—,—CO—, —O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—,—CH═CH— or —C≡C—.

Typical spacer groups Sp are for example —(CH₂)_(o)—,—(CH₂CH₂O)_(r)—CH₂CH₂—, —CH₂CH₂—S—CH₂CH₂— or —CH₂CH₂—NH—CH₂CH₂—,with obeing an integer from 2 to 12 and r being an integer from 1 to 3.

Preferred spacer groups Sp are ethylene, propylene, butylene, pentylene,hexylene, heptylene, octylene, nonylene, decylene, undecylene,dodecylene, octadecylene, ethyleneoxyethylene, methyleneoxybutylene,ethylene-thioethylene, ethylene-N-methyl-iminoethylene and1-methylalkylene, for example.

In the event that R or Q² is a group of formula P-Sp-X— or P-Sp-respectively, the spacer groups on each side of the mesogenic core maybe identical or different.

In particular preferred are compounds of formula I wherein n is 1.

In another preferred embodiment, the inventive optical retardation filmis obtained by copolymerizing mixtures comprising compounds of formula Iwherein n is 0 and compounds of formula I wherein n is 1.

Typical examples representing polymerizable mesogenic compounds of theformula I can be found in WO 93/22397; EP 0,261,712; DE 195,04,224; DE4,408,171 or DE 4,405,316. The compounds disclosed in these documents,however are to be regarded merely as examples that should not limit thescope of this invention.

Furthermore, typical examples representing polymerizable mesogeniccompounds are shown in the following list of compounds, which is,however, to be understood only as illustrative without limiting thescope of the present invention:

In these compounds x and y are each independently 1 to 12, A is a1,4-phenylene or 1,4-cyclohexylene group, R¹ is halogen, cyano or analkyl or alkoxy group with 1 to 12 C atoms and L¹ and L² are eachindependently H, Halogen, CN, or an alkyl, alkoxy or alkanoyl group with1 to 7 C atoms.

The reactive mesogenic compounds disclosed in the foregoing and thefollowing can be prepared by methods which are known per se and whichare described in the documents cited above and, for example, in standardworks of organic chemistry such as, for example, Houben-Weyl, Methodender organischen Chemie, Thieme-Verlag, Stuttgart.

In a preferred embodiment of the present invention, the opticalretardation film is obtainable from a mixture of a polymerizablemesogenic material comprising the following components

-   -   a1) 15 to 95%, preferably 20 to 90% by weight of at least one        mesogen according to formula I having one polymerizable        functional group,    -   a2) 5 to 80%, preferably 8 to 70%, in particular 10 to 50% by        weight of at least one mesogen according to formula I having two        or more polymerizable functional groups,    -   b) 0.01 to 5% by weight of an initiator,    -   c) 0 to 20% by weight of a non-mesogenic compound having two or        more polymerizable functional groups,    -   d) 0 to 1000 ppm of a stabilizer, and    -   e) 0 to 5% by weight of a chain transfer agent.

Mixtures according to this particularly preferred embodiment arepreferred that comprise one to eight, in particular one to six, mostpreferably one to three different mesogens according to formula I havingone polymerizable functional group.

The mixture according to this particularly preferred embodimentespecially preferably contains less than 10% by weight, very especiallypreferably none of the compounds of component c).

In another embodiment of the present invention, the mixture of thepolymerizable mesogenic material comprises

-   -   a1) 15 to 99%, preferably 40 to 99%, in particular 70 to 99% by        weight of at least one mesogen according to formula I having one        polymerizable functional group,    -   a2) 0 to 90% by weight of at least one mesogen according to        formula I having two or more polymerizable functional groups,    -   b) 0.01 to 5% by weight of an initiator,    -   c) 0 to 20% by weight of a non-mesogenic compound having two or        more polymerizable functional groups,    -   d) 0 to 1000 ppm of a stabilizer, and    -   e) 0 to 5% by weight of a chain transfer agent.

Mixtures according to this particularly preferred embodiment arepreferred that comprise one to eight, in particular one to six, mostpreferably one to three different mesogens according to formula I havingone polymerizable functional group.

Further preferred are mixtures according this preferred embodiment thatcomprise 15 to 99% by weight of at least two different mesogens ofcomponent al) and further comprises components b) and optionallycomponent a2), c), d) and e) as described above.

Mixtures according to this particularly preferred embodiment arepreferred that comprise two to eight, in particular two to six, mostpreferably two to four different mesogens according to formula I havingone polymerizable functional group.

Further preferred are mixtures according to this particularly preferredembodiment that comprise four or more, in particular four to eight, veryparticularly four to six different mesogens according to formula Ihaving one polymerizable functional group.

The ratio of each of the mesogens according to formula I having onepolymerizable functional group in the mixture according to thisparticularly preferred embodiment is preferably 5 to 90%, in particular10 to 80%, very preferably 15 to 65% by weight of the total mixture.

The mixture according to this particularly preferred embodimentespecially preferably contains less than 10% by weight, very especiallypreferably none of the compounds of component a2).

In the mixtures comprising two or more different mesogens according toformula I having one polymerizable functional group as described above,preferably each of the different mesogens according to formula I isdifferent in at least one of the groups P, Sp, X, A¹, A², A³, Z¹, Z² andR from each other of the mesogens.

The mixtures of a polymerizable mesogenic material as described aboveare another object of the present invention.

Without further elaboration one skilled in the art can, using thepreceding description, utilize the present invention to its fullestextent. The following examples are, therefore, to be construed as merelyillustrative and not limitative of the remainder of the disclosure inany way whatsoever.

In the foregoing and in the following examples, unless otherwiseindicated, all temperatures are set forth uncorrected in degrees Celsiusand all parts and percentages are by weight. The following abbreviationsare used to illustrate the liquid crystalline phase behavior of thecompounds:

K=crystalline; N=nematic; S=smectic; Ch=cholesteric; I=isotropic. Thenumbers between these symbols indicate the phase transition temperaturesin degree Celsius.

EXAMPLE 1

The following mixture was formulated

The compounds (1) and (2) have been prepared in analogy to the methodsdescribed in WO 93/22397 and DE195,04,224. Irgacure 651 is aphotoinitiator for radicalic polymerization which is commerciallyavailable from Ciba Geigy AG.

To prepare crosslinked polymer films, the mixture was dissolved intoluene at a concentration of about 20% by weight and filtered to removeimpurities and small particles.

A sheet of PET (Melinex 401, available from ICI Corp.) was rubbedunidirectionally 50 to 60 times with a flat aluminium bar coated withvelvet. The applied pressure was approximately 2 g/cm³, and the rubbinglength was approximately 1.5±0.2 meters.

The toluene mixture was coated as a film with a thickness ofapproximately 12 μm on the PET sheet and the solvent was allowed toevaporate at 55° C. The mixture was then cured in a nitrogen atmosphereat 55° C. by irradiating with UV light with a wavelength of 350 to 380nm and an irradiance of 40 mW/cm² for 4 minutes.

In this way, two-crosslinked polymer films (1 a, 1 b) with differentthickness were obtained that can be used as a retardation film.

EXAMPLE A

The films 1 a and 1 b obtained as described in example 1 were removedfrom the PET substrate and their retardation was measured on a glassslide on an Olympus polarizing microscope using a Berek compensator. Thefilm 1 a has a retardation value of 134 nm, and the film 1 b aretardation value of 154 nm.

FIG. 2 shows the change of the retardation depending on the viewingangle for a sample of the inventive optical quarter wave retardationfilms 1 a (curve 2 a) and 1 b (curve 2 b) in comparison to a standardQWF based on PVA (curve 2 c). It can be clearly seen that the viewingangle dependence of both inventive optical quarter wave retardationfilms is lower than that of the PVA film.

EXAMPLE B

The optical performance of the inventive retardation film was determinedin an inventive optical combination together with a broadbandcholesteric reflective polarizer.

The broad waveband reflective polarizer film consisted of a polymerizedmixture comprising chiral and achiral reactive mesogenic compounds. Thepolarizer exhibited a cholesteric structure with planar orientation withmultiple pitch lengths of the cholesteric helix and had a broadwavelength reflection band as shown in FIG. 4 which is ranging fromwavelength values of 500 to 800 nm with a bandwidth of about 300 nm.

The retardations of the films 1 a (134 nm) and 1 b (154 nm) are 0.25times a value lying inside the waveband reflected by the broadbandreflective polarizer, therefore each of these two films, when usedtogether with the reflective polarizer, can act as a quarter wave film.

FIG. 3 depicts the measurement setup. The luminance of light from acommercial LCD backlight 50 passing through an assembly with thereflective polarizer 51 and the inventive optical retardation films 1 aand 1 b of example 1 (52) was measured using a Minolta CS-100 colorcamera 53 at a range of viewing angles (−60° to +60°). The experimentwas repeated with a similar assembly, wherein the inventive opticalquarter wave retardation films were replaced by the standard QWF basedon PVA as used in example A. The measurement results are shown in FIG.5.

Curve 5 x depicts the luminance of the LCD backlight 50 together withthe reflective polarizer 51. Curves 5 a, 5 b and 5 c show the luminanceof the LCD backlight 50 and a combination of the reflective polarizer 51together with an optical retardation film 52 that is either one of theinventive optical retardation films 1 a (curve 5 a) or 1 b (curve 5 b)or the standard QWF (curve 5 c).

The luminance of the assembly comprising the inventive opticalretardation films 1 a (curve 5 a) and 1 b (curve 5 b) is higher thanthat of the assembly comprising the QWF based on PVA (curve 5 c) overthe whole range of measured viewing angles, and the cross-over angleα_(c) is increased by about 5 to 6 degrees.

The optical retardation film 52 and the broadband reflective polarizer51 in FIG. 3 according to example B were not optically coupled. If theyare laminated together, or if the reflective polarizer is prepared bypolymerization of a mixture of reactive cholesteric mesogenic compoundsusing the optical retardation film as a substrate, the cross over angleα_(c) is being further increased.

The results of experiments according to example A and B clearlydemonstrate the improved properties of an inventive optical retardationfilm compared to an optical retardation film of the state of the art,especially when used in combination with a broadband cholestericreflective polarizer.

EXAMPLE C

A PET web substrate for planar alignment was prepared by rubbingaccording to a preferred embodiment of the present invention. Rubbingwas carried out as depicted in FIG. 6 by wrapping the PET web 1 at awrap angle a of 65° around a velvet coated roller 2 with a circumferenceof 125 cm, which was rotating with a velocity of 300 rpm. The web speedwas 1750 cm/min and the tension on the web was 5 lbs/inch. This resultedin a rubbing length of 1335 mm. An inventive polymerizable mesogenicmixture coated on this PET substrate showed planar alignment of a highuniformity with substantially no tilt.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting form the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A method of preparing a film comprising at least one layer of ananisotropic polymer material, comprising polymerizing a mixture of afiltered polymerizable mesogenic material dissolved in an organicsolvent.
 2. A method according to claim 1, wherein the polymerizablemesogenic material is filtered through a filter before polymerization.3. A method according to claim 1, wherein the polymerizable mesogenicmaterial comprises: at least one reactive mesogen having at least onepolymerizable functional group; an initiator; optionally a non-mesogeniccompound having two or more polymerizable functional groups; andoptionally a stabilizer.
 4. A method according to claim 1, wherein thepolymerizable mesogenic material comprises one or more reactive mesogensof the formula I:P-(Sp-X)_(n)-MG-R   I wherein P is a polymerizable group; Sp is a spacergroup having 1 to 20 C atoms; X is a group selected from —O—, —S—, —CO—,—COO—, —OCO—, —OCOO—, or a single bond; R is an unsubstituted alkylradical with up to 25 C atoms optionally mono- or polysubstituted byhalogen or CN, optionally one or more non-adjacent CH₂ groups arereplaced, in each case independently from one another, by —O—, —S—,—NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— insuch a manner that oxygen atoms are not linked directly to one another,or alternatively, R is halogen, cyano or has independently one of themeanings given for P-(Sp-X)_(n)—; n is 0 or 1; and MG is a mesogenic ormesogenity supporting group.
 5. A method according to claim 1, whereinthe concentration of the solvent is at least 20% of the totalpolymerizable mixture.
 6. A method according to claim 1, wherein thepolymerizable mesogenic material comprises one or more surface activecompounds.
 7. A polymerizable mesogenic material made according to claim3, for the preparation of an anisotropic polymer film, wherein thematerial is filtered before polymerization.
 8. A film comprising atleast one layer of anisotropic polymer material prepared by a methodaccording to claim
 1. 9. An optical retardation film comprising a filmaccording to claim
 8. 10. A method according to claim 4, wherein MG isof the formula II:-(A¹-Z¹)_(m)-A²Z²-A³-   II wherein A¹, A², and A³ are independently fromeach other 1,4-phenylene wherein, optionally, one or more CH groups arereplaced by N; 1,4-cyclohexylene wherein, optionally, one or twonon-adjacent CH₂ groups are replaced by O and/or S; an unsubstituted1,4-cyclohexenylene or unsubstituted naphthalene-2,6-diyl, or optionallythe 1,4-cyclohexenylene or naphthalene-2,6-diyl are mono- orpolysubstituted with halogen, cyano, or nitro groups, with alkyl, alkoxyor alkanoyl groups having 1-7 C atoms wherein one or more H atoms areoptionally replaced with F or Cl; Z¹ and Z² are each independently—COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—,—OCO—CH═CH—, or a single bond, and m is 0, 1, or
 2. 11. A methodaccording to claim 1, wherein the organic solvent is toluene.
 12. Amethod according to claim 4, wherein the mesogens are bicyclic ortricyclic compounds.
 13. A method according to claim 4, wherein R is F,Cl, cyano, alkyl, alkoxy, or P-(Sp-X)_(n)—.
 14. A method according toclaim 13, wherein Z¹ and Z² are, independently, —COO—, —OCO—, —CH₂—CH₂—,—CH═CH—COO—, —OCO—CH═CH—, or a single bond.
 15. A method according toclaim 4, wherein P is a vinyl group, an acrylate group, a methacrylategroup, a propenyl ether group or an epoxy group.
 16. A method accordingto claim 4, wherein Sp is a linear or branched alkylene group of 1-20 Catoms.
 17. A method of preparing a film comprising at least one layer ofan anisotropic polymer material, comprising polymerizing a mixture of afiltered polymerizable mesogenic material wherein the polymerizablemesogenic material comprises one or more surface active compounds.
 18. Amethod according to claim 17, wherein the polymerizable mesogenicmaterial is filtered through a filter before polymerization.
 19. Amethod according to claim 17, wherein the polymerizable mesogenicmaterial comprises: at least one reactive mesogen having at least onepolymerizable functional group; an initiator; optionally a non-mesogeniccompound having two or more polymerizable functional groups; andoptionally a stabilizer.
 20. A method according to claim 17, wherein thepolymerizable mesogenic material comprises one or more reactive mesogensof the formula I:P-(Sp-X)_(n)-MG-R   I wherein P is a polymerizable group; Sp is a spacergroup having 1 to 20 C atoms; X is a group selected from —O—, —S—, —CO—,—COO—, —OCO—, —OCOO—, or a single bond; R is an unsubstituted alkylradical with up to 25 C atoms optionally mono- or polysubstituted byhalogen or CN, optionally one or more non-adjacent CH₂ groups arereplaced, in each case independently from one another, by —O—, —S—,—NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— insuch a manner that oxygen atoms are not linked directly to one another,or alternatively, R is halogen, cyano or has independently one of themeanings given for P-(Sp-X)_(n)—; n is 0 or 1; and MG is a mesogenic ormesogenity supporting group.
 21. A method according to claim 17, whereinthe polymerizable mesogenic material is dissolved in an organic solvent.22. A method according to claim 21, wherein the concentration of thesolvent is at least 20% of the total polymerizable mixture.
 23. Apolymerizable mesogenic material made according to claim 19, for thepreparation of an anisotropic polymer film, wherein the material isfiltered before polymerization.
 24. A film comprising at least one layerof anisotropic polymer material prepared by a method according to claim17.
 25. An optical retardation film comprising a film according to claim24.
 26. A method according to claim 20, wherein MG is of the formula II:-(A¹-Z¹)_(m)-A²-Z²-A³-   II wherein A¹, A², and A³ are independentlyfrom each other 1,4-phenylene wherein, optionally, one or more CH groupsare replaced by N; 1,4-cyclohexylene wherein, optionally, one or twonon-adjacent CH₂ groups are replaced by O and/or S; an unsubstituted1,4-cyclohexenylene or unsubstituted naphthalene-2,6-diyl, or optionallythe 1,4-cyclohexenylene or naphthalene-2,6-diyl are mono- orpolysubstituted with halogen, cyano, or nitro groups, with alkyl, alkoxyor alkanoyl groups having 1-7 C atoms wherein one or more H atoms areoptionally replaced with F or Cl; Z¹ and Z² are each independently—COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—,—OCO—CH═CH—, or a single bond, and m is 0, 1, or
 2. 27. A methodaccording to claim 21, wherein the organic solvent is toluene.
 28. Amethod according to claim 20, wherein the mesogens are bicyclic ortricyclic compounds.
 29. A method according to claim 20, wherein R is F,Cl, cyano, alkyl, alkoxy, or P-(Sp-X)_(n)—.
 30. A method according toclaim 26, wherein Z¹ and Z² are, independently, —COO—, —OCO—, —CH₂—CH₂—,—CH═CH—COO—, —OCO—CH═CH—, or a single bond.
 31. A method according toclaim 20, wherein P is a vinyl group, an acrylate group, a methacrylategroup, a propenyl ether group or an epoxy group.
 32. A method accordingto claim 20, wherein Sp is a linear or branched alkylene group of 1-20 Catoms.
 33. A polymerizable material, comprising a mixture of a filteredpolymerizable mesogenic material dissolved in an organic solvent.
 34. Apolymerizable material according to claim 33, wherein the polymerizablemesogenic material comprises: at least one reactive mesogen having atleast one polymerizable functional group; an initiator; optionally anon-mesogenic compound having two or more polymerizable functionalgroups; and optionally a stabilizer.
 35. A polymerizable materialaccording to claim 33, wherein the polymerizable mesogenic materialcomprises one or more reactive mesogens of the formula I:P-(Sp-X)_(n)-MG-R   I wherein P is a polymerizable group; Sp is a spacergroup having 1 to 20 C atoms; X is a group selected from —O—, —S—, —CO—,—COO—, —OCO—, —OCOO—, or a single bond; R is an unsubstituted alkylradical with up to 25 C atoms optionally mono- or polysubstituted byhalogen or CN, optionally one or more non-adjacent CH₂ groups arereplaced, in each case independently from one another, by —O—, —S—,—NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— insuch a manner that oxygen atoms are not linked directly to one another,or alternatively, R is halogen, cyano or has independently one of themeanings given for P-(Sp-X)_(n)—; n is 0 or 1; and MG is a mesogenic ormesogenity supporting group.
 36. A polymerizable material according toclaim 35, wherein MG is of the formula II:-(A¹-Z¹)_(m)-A²Z²-A³-   II wherein A¹, A², and A³ are independently fromeach other 1,4-phenylene wherein, optionally, one or more CH groups arereplaced by N; 1,4-cyclohexylene wherein, optionally, one or twonon-adjacent CH₂ groups are replaced by O and/or S; an unsubstituted1,4-cyclohexenylene or unsubstituted naphthalene-2,6-diyl, or optionallythe 1,4-cyclohexenylene or naphthalene-2,6-diyl are mono- orpolysubstituted with halogen, cyano, or nitro groups, with alkyl, alkoxyor alkanoyl groups having 1-7 C atoms wherein one or more H atoms areoptionally replaced with F or Cl; Z¹ and Z² are each independently—COO—, —OCO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—,—OCO—CH═CH—, or a single bond, and m is 0, 1, or
 2. 37. A polymerizablematerial according to claim 33, wherein the organic solvent is toluene.38. A polymerizable material according to claim 35, wherein the mesogensare bicyclic or tricyclic compounds.
 39. A polymerizable materialaccording to claim 35, wherein R is F, Cl, cyano, alkyl, alkoxy, orP-(Sp-X)_(n)—.
 40. A polymerizable material according to claim 36,wherein Z¹ and Z² are, independently, —COO—, —OCO—, —CH₂—CH₂—,—CH═CH—COO—, —OCO—CH═CH—, or a single bond.
 41. A polymerizable materialaccording to claim 35, wherein P is a vinyl group, an acrylate group, amethacrylate group, a propenyl ether group or an epoxy group.
 42. Apolymerizable material according to claim 35, wherein Sp is a linear orbranched alkylene group of 1-20 C atoms.
 43. A polymerizable materialcomprising a mixture of a filtered polymerizable mesogenic material andone or more surface active compounds.
 44. A polymerizable materialaccording to claim 43, wherein the polymerizable mesogenic materialcomprises: at least one reactive mesogen having at least onepolymerizable functional group; an initiator; optionally a non-mesogeniccompound having two or more polymerizable functional groups; andoptionally a stabilizer.
 45. A polymerizable material according to claim43, wherein the polymerizable mesogenic material comprises one or morereactive mesogens of the formula I:P-(Sp-X)_(n)-MG-R   I wherein P is a polymerizable group; Sp is a spacergroup having 1 to 20 C atoms; X is a group selected from —O—, —S—, —CO—,—COO—, —OCO—, —OCOO—, or a single bond; R is an unsubstituted alkylradical with up to 25 C atoms optionally mono- or polysubstituted byhalogen or CN, optionally one or more non-adjacent CH₂ groups arereplaced, in each case independently from one another, by —O—, —S—,—NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—, —CO—S— or —C≡C— insuch a manner that oxygen atoms are not linked directly to one another,or alternatively, R is halogen, cyano or has independently one of themeanings given for P-(Sp-X)_(n)—; n is 0 or 1; and MG is a mesogenic ormesogenity supporting group.
 46. A polymerizable material according toclaim 45, wherein MG is of the formula II:-(A¹-Z¹)_(m)-A²Z²-A³-   II wherein A¹, A², and A³ are independently fromeach other 1,4-phenylene wherein, optionally, one or more CH groups arereplaced by N; 1,4-cyclohexylene wherein, optionally, one or twonon-adjacent CH₂ groups are replaced by O and/or S; an unsubstituted1,4-cyclohexenylene or unsubstituted naphthalene-2,6-diyl, or optionallythe 1,4-cyclohexenylene or naphthalene-2,6-diyl are mono- orpolysubstituted with halogen, cyano, or nitro groups, with alkyl, alkoxyor alkanoyl groups having 1-7 C atoms wherein one or more H atoms areoptionally replaced with F or Cl; Z¹ and Z² are each independently—COO—, —OCO—, —CH₂CH₂—, —OCH₂—, ‘3CH₂O—, —CH═CH—, —C≡C—, —CH═CH—COO—,—OCO—CH═CH—, or a single bond, and m is 0, 1, or
 2. 47. A polymerizablematerial according to claim 43, wherein the filtered polymerizablemesogenic material is dissolved in toluene.
 48. A polymerizable materialaccording to claim 45, wherein the mesogens are bicyclic or tricycliccompounds.
 49. A polymerizable material according to claim 45, wherein Ris F, Cl, cyano, alkyl, alkoxy, or P-(Sp-X)_(n)—.
 50. A polymerizablematerial according to claim 46, wherein Z¹ and Z² are, independently,—COO—, —OCO—, —CH₂—CH₂—, —CH═CH—COO—, —OCO—CH═CH—, or a single bond. 51.A polymerizable material according to claim 45, wherein P is a vinylgroup, an acrylate group, a methacrylate group, a propenyl ether groupor an epoxy group.
 52. A polymerizable material according to claim 45,wherein Sp is a linear or branched alkylene group of 1-20 C atoms.
 53. Apolymerizable material according to claim 33, wherein the polymerizablemesogenic material comprises one or more surface active compounds.